The present invention relates generally to a system for compiling an image of a wellbore wall with a bottomhole drilling assembly. More particularly, the invention relates to a bottomhole drilling assembly that includes a logging while drilling ("LWD") sub-system for determining characteristics of the borehole and formation during the drilling of a well, and correlating that information with depth to produce an image of some desired portion of the borehole. Still more particularly, the present invention relates to a device that provides an image of the borehole as part of an LWD tool, and which also can be used as a device to determine other formation parameters such as formation dip angle.
Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, in addition to data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as "logging," can be performed by several methods. Logging has been known in the industry for many years as a technique for providing information regarding the particular earth formation being drilled. In conventional oil well wireline logging, a probe or "sonde" is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The sonde may include one or more sensors to measure parameters downhole and typically is constructed as a hermetically sealed steel cylinder for housing the sensors, which hangs at the end of a long cable or "wireline." The cable or wireline provides mechanical support to the sonde and also provides an electrical connection between the sensors and associated instrumentation within the sonde, and electrical equipment located at the surface of the well. Normally, the cable supplies operating power to the sonde and is used as an electrical conductor to transmit information signals from the sonde to the surface, and control signals from the surface to the sonde. In accordance with conventional techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole, as the sonde is pulled uphole.
The sensors used in a wireline sonde may include a source device for transmitting energy into the formation, and one or more receivers for detecting the energy reflected from the formation. Various sensors have been used to determine particular characteristics of the formation, including nuclear sensors, acoustic sensors, and electrical sensors. See generally J. Labo, A Practical Introduction to Borehole Geophysics (Society of Exploration Geophysicists 1986); D. R. Skinner, Introduction to Petroleum Production, Volume 1, at 54-63 (Gulf Publishing Co. 1981).
For an underground formation to contain petroleum, and for the formation to permit the petroleum to flow through it, the rock comprising the formation must have certain well known physical characteristics. One characteristic is that the rock in the formation have space to store petroleum. If the rock in a formation has openings, voids, and spaces in which oil and gas may be stored, it is characterized as "porous." Thus, by determining if the rock is porous, one skilled in the art can determine whether or not the formation has the requisite physical properties to store and yield petroleum. See D. R. Skinner, Introduction to Petroleum Production, id. at 8.
Acoustic sensors are commonly used to measure porosity of the formation by determining the amount of time it takes the acoustic wave to travel through the formation. The porosity of the formation through which the acoustic wave travels influences the speed of sound in that formation. By determining the speed of sound of a formation, valuable insight can be obtained regarding formation porosity and other formation characteristics. Examples of acoustic wireline tools are U.S. Pat. Nos. 3,237,153, 3,312,934, 3,593,255, 4,649,525, 4,718,046, 4,869,349, and 5,069,308. Typically, the acoustic wireline tools include one or more acoustic transmitters and one or more acoustic receivers. Acoustic waves are generated by the transmitter(s) and are transmitted into the formation adjacent the wellbore. The acoustic signals are refracted back to the receivers, and a travel time for the wave is determined, typically at the surface of the well. From this travel time, and knowing the spacing between the receivers, speed of sound of the formation can be calculated, which then can be used to provide an indication of formation porosity according to known techniques. See generally J. Labo, A Practical Introduction to Borehole Geophysics, Chapter 10 (Society of Exploration Geophysicists 1986).
Acoustic logging tools also have been used by the assignee of the present invention as a wireline imaging device. See Open Hole Services, (Halliburton Logging Services 1992), p. 28. This device is commonly referred to as the Circumferential Acoustic Scanning Tool (or CAST). As the sonde is pulled up the borehole, an acoustic transducer is rotated at a high speed (on the order of 10 revolutions per second). During each rotation, the acoustic transducer is fired based upon its relative rotational bearing. Thus, the radial directions are divided into a plurality of finite points (on the order of 200 points), and the transducer is fired at each point. Reflections are received by the transducer, and the amplitude and time-of-flight of the reflected signal is computed and transmitted via the wireline to the surface for processing. If the transducer is fired 200 times per revolution, and the transducer is rotated 10 times per second, then the transducer is fired 2000 times per second, and receives 2000 reflected signals in one second. The reflected signals from the wireline imaging device are used to produce rotational image logs, as well as borehole caliper measurements and/or standoff measurements using two-way time-of-flight of a pulse-echo signal (with an assumed mud slowness).
While wireline logging is useful in assimilating information relating to formations downhole, it nonetheless has certain disadvantages. For example, before the wireline logging tool can be run in the wellbore, the drillstring and bottomhole assembly must first be removed or tripped from the borehole, resulting in considerable cost and loss of drilling time for the driller (who typically is paying daily fees for the rental of drilling equipment). In addition, because wireline tools are unable to collect data during the actual drilling operation, drillers must at times make decisions (such as the direction to drill, etc.) possibly without sufficient information, or else incur the cost of tripping the drillstring to run a logging tool to gather more information relating to conditions downhole. In addition, because wireline logging occurs a relatively long period after the wellbore is drilled, the accuracy of the wireline measurement can be questionable. As one skilled in the art will understand, the wellbore conditions tend to degrade as drilling fluids invade the formation in the vicinity of the wellbore. In addition, the borehole shape may begin to degrade, reducing the accuracy of the measurements.
Because of these limitations associated with wireline logging, there recently has been an increasing emphasis on the collection of data during the drilling process itself. By collecting and processing data during the drilling process, without the necessity of tripping the drilling assembly to insert a wireline logging tool, the driller can make accurate modifications or corrections "real-time", as necessary, to optimize drilling performance. For example, the driller may change the weight-on-bit to cause the bottomhole assembly to tend to drill in a particular direction, or, if a steerable bottomhole assembly is used, may operate in the sliding mode to effect source corrections. Moreover, the measurement of formation parameters during drilling, and hopefully before invasion of the formation, increases the usefulness of the measured data. Further, making formation and borehole measurements during drilling can save the additional rig time which otherwise would be required to run a wireline logging tool.
Designs for measuring conditions downhole and the movement and location of the drilling assembly, contemporaneously with the drilling of the well, have come to be known as "measurement-while-drilling" techniques, or "MWD." Similar techniques, concentrating more on the measurement of formation parameters of the type associated with wireline tools, commonly have been referred to as "logging while drilling" techniques, or "LWD." While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term LWD will be used with the understanding that the term encompasses both the collection of formation parameters and the collection of information relating to the position of the drilling assembly while the bottomhole assembly is in the well.
The measurement of formation properties during drilling of the well by LWD systems improves the timeliness of measurement data and, consequently, increases the efficiency of drilling operations. Typically, LWD measurements are used to provide information regarding the particular formation in which the borehole is traversing. During the last several years, many in the industry have noted the desirability of an LWD system that could be especially used to detect bed boundaries and to provide real-time data to the driller to enable the driller to make directional corrections to stay in the pay zone. See, e.g. Olsen, "Potential of MWD Tools," Euroil, March 1993, pp. 17-18. Alternatively, the LWD system could be used as part of a "Smart" system to automatically maintain the drill bit in the pay zone. See, e.g. commonly assigned U.S. Pat. No. 5,332,048, the teachings of which are incorporated by reference herein. The assignee has also developed a system which permits the measurement of LWD data at the drill bit to provide an earlier indication of bed boundaries and formation characteristics. See U.S. Pat. No. 5,160,925. The use of an LWD system with these other systems makes it possible to conduct at least certain portions of drilling automatically.
Currently, logging sensors that commonly are used as part of an LWD system are resistivity and gamma ray sensors. Some in the industry have begun offering imaging systems based upon resistivity measurements. See the IDEAL system brochures published by Schlumberger. Acoustic measurement devices currently are being experimented with and implemented by the assignee and other companies for use in LWD systems to calculate a correction factor for other LWD measurements, and to determine formation speed of sound. See, e.g. commonly assigned U.S. patent application Ser. No. 08/430,822, now allowed entitled "Standoff Compensation for Acoustic Logging While Drilling Systems." The primary goal of acoustic measurements in LWD systems is to develop additional data relating to formation porosity by determining the formation speed of sound, which corresponds to measurements obtained by wireline logging. See "Sonic While Drilling--Real-Time Data to Guide Real-Time Decisions," Schlumberger Oilfield Services catalogue. The implementation of acoustic logging tools in a LWD system, however, is complicated as compared to a wireline tool because of the presence of extraneous drilling noise downhole. Thus, the noise generated by the drilling assembly, the flow of mud through the drillstring, the grinding of the drilling components, and other mechanical and environment noises present downhole make it difficult for the acoustic transducers to receive the transmitted acoustic wave and to isolate the received acoustic waveform from the extraneous noise that also is detected by the acoustic receiver. As a result, to date, acoustic sensors have had a very limited application in LWD systems.
Until very recently, imaging devices only have been available in wireline logging tools. Imaging devices have not been used extensively downhole for a myriad of reasons. One problem with implementing an LWD imaging tool is that the images must be stored downhole, and cannot be transmitted to the surface as quickly as is done in wireline logging because of the manner in which data is transmitted in LWD systems. The images require a massive amount of memory storage space. A second problem with LWD imaging relates to depth measurements. In wireline logging, depth can be tracked with better resolution than is available in the drilling process because more precise depth measurements are available for use with wireline devices than with LWD systems. Because of the problem with depth measurements during drilling, there is a great concern that LWD images will be smeared, or must be time correlated instead of depth correlated. A third concern with LWD imaging relates to the limitations on processing downhole, which results in an under-utilization of the imaged data until it can be retrieved at the surface.
While the advantages of determining borehole images during drilling are immediately apparent to one skilled in the art, to date no one has successfully implemented such a system which overcomes the limitations listed above.